How to remove (unlink) all the grease pencil layers?
Short answer: 
Yes!

Press N, go to Grease Pencil and uncheck the checkbox:



This will hide all grease pencil sketches, while retaining the information in case you want your sketches back at a later time.

Long answer: 
In the Viewport press N and go to the Grease Pencil portion of the menu on the right. 



There are several ways to remove a grease pencil sketch, you can just hide the grease pencil (the checkbox), you can delete a frame, you can delete a layer (the "X"), or you can hide a layer (the eye).
You can use the RMB or use Shift+D to get the grease eraser. Use the LMB (or RMB depending on the method) to erase, and the MMB to change the size of the brush.